Closed-loop pre-coding technology has been introduced in the Long Term Evolution (LTE) Release 8 (Rel-8) system to improve the spectrum efficiency. Closed-loop pre-coding essentially requires the same set of pre-coding matrixes, referred to as a codebook, to be stored at both a base station and a User Equipment (UE). The UE estimates channel information according to a cell common pilot and then selects a pre-coding matrix from the codebook according to some criterion, which can be the maximizing mutual information, the maximizing output signal to interference and noise ratio, etc. The UE feeds an index of the selected pre-coding matrix in the codebook to the base station through an uplink channel, where the index is referred to as a Pre-coding Matrix Indicator (PMI). The base station can determine from the value of the received index the pre-coding matrix to be used for the UE. The pre-coding matrix reported by the UE can be considered as a quantized value of channel state information.
A Rank Indicator (RI) and a Channel Quality Indicator (CQI) will be reported by the UE in addition to the PMI, where the RI corresponds to the number of separate data streams in spatially multiplexed transmission, that is, the RI indicates the largest number of spatial data streams which can be supported over spatial channels from the base station to the UE, and the CQI indicates the quality of the channel from the base station to the UE as a criterion to select a modulation scheme and a coding rate. In the LTE and Long Term Evolution-Advanced (LTE-A) specification, a data block transmitted by the base station to the UE can be referred to as a codeword, and the base station can transmit at most two codewords concurrently to the UE, or may transmit only one codeword thereto, in spatial multiplexing. Data of a codeword are mapped onto one or more spatial data streams under some rule. If two codewords are transmitted, then the total number of data streams to which the two codewords are mapped shall not exceed the largest number of data streams which can be supported over the spatial channels from the base station to the UE, where the number is derived from the RI reported by the UE. If transmission of multiple data streams can be supported over the spatial channels, i.e., RI>1, which means that two codewords are transmitted by the base station to the UE, then the UE will calculate CQIs of the two codewords respectively and feed back the CQIs.
Different codebooks are designed in the LTE and the LTE-A for transmission of different numbers of data streams, so that the base station needs to determine the number of data streams according to the RI fed back by the UE to thereby determine from which of the codebooks the pre-coding matrix shall be selected, and the PMI indicates the index of the pre-coding matrix recommended by the UE in the corresponding codebook. Thus the pre-coding matrix recommended by the UE to the base station is determined by both the RI and the PMI fed back by the UE. The CQI corresponds to channel quality information under the condition that the base station applies the pre-coding matrix recommended by the UE.
In the existing cellular system, an array of antennas of the base station is typically arranged horizontally as illustrated in FIG. 1 and FIG. 2. A beam at a transmitting end, the base station, can only be adjusted horizontally with a common vertical down tilt angle, so various beam-forming/pre-coding technologies are overallly implemented based upon horizontal channel information. In fact, since a radio signal propagates in three dimensions in a space, the performance of the system can not be optimized with this common vertical down tilt angle. Adjusting of the beam in vertical direction will be of great significance to lowering inter-cell interference and improving the performance of the system. As the antenna technologies are developing, an array of active antennas in which array elements can be controlled separately has emerged in the industry as illustrated in FIG. 3A and FIG. 3B.
For the array of active antennas in which each of array elements is controlled separately, the network-side device needs to determine an overall pre-coding matrix, but there has been absent so far a solution to determine the overall pre-coding matrix.